1. Field of the Invention
The present invention relates to a vari-focal lens system and, more particularly, to a vari-focal lens system with a large aperture ratio and high vari-focal ratio.
2. Description of the Prior Art
Recently, still cameras and video cameras provided with automatic focusing systems are drawing attention because they make it possible to omit the focusing operation to be performed by the operator. The largest merit of automatic focusing is a high focusing speed. The focusing speed is decided by the type and circuit of the automatic focusing system, torque of the drive motor, weight of lenses to be moved for focusing, moving distances of those lenses, etc. Therefore, it is preferable to arrange the lens system so that the lens group to be moved for focusing is as light as possible and the amount of movement thereof is as small as possible.
Besides, lately provided known vari-focal lens systems show such tendency that both of the number of lens groups to be moved when varying the focal length of the lens system and number of lens groups to be moved at the time of focusing become larger and, therefore, they have such disadvantages that the lens moving mechanism thereof becomes complicated and the cost of production becomes high.
For example, most of known vari-focal lens systems are arranged that at least three lens groups should be moved for focusing and for varying the focal length. Some of known vari-focal lens systems are arranged that only two lens groups are moved for varying the focal length and for focusing. However, those known vari-focal lens systems have a disadvantage that the front lens group with a comparatively large lens diameter is to be moved.
Here, let us discuss the problems about vari-focal lens systems for cameras wherein an electronic image pick-up tube or solid-state image sensor is used as the image pick-up device. The above-mentioned kind of known vari-focal lens systems provided lately are arranged that the vari-focal ratio is 3 to 8, the focal length in the wide position is approximately equal to the length of the diagonal line of the image surface, the sensitivity of the image sensor is low and, therefore, the F-number is smaller than 2.0, i.e., the lens system is bright. Said known lens systems are generally arranged to comprise four lens groups, i.e., a focusing lens group, a focal length varying lens group, a compensating lens group for compensating the displacement of the focal point (so-called compensator), and a relay lens group to be kept fixed, and arranged that the focusing lens group is moved at the time of focusing while the focal length varying lens group and compensating lens group are moved along the optical axis when varying the focal length. Said vari-focal lens systems have problems described below. That is, as the focusing lens group is heavy, the response at the time of focusing becomes low and the power consumption becomes large. Moreover, the eclipse of offaxial rays becomes larger when the focusing lens group is advanced at the time of focusing of an object at a short distance. Therefore, it is necessary to make the front lens diameter still larger, and this is more disadvantageous.
To solve the above-mentioned problem, some of known vari-focal lens systems are arranged to be focused by moving the lens groups located in the rear part of the lens system whose lens diameters are small. However, in case of the above-mentioned known lens systems, the number of lens groups to be moved for focusing and for varying the focal length is large, i.e., three of more. The lens system disclosed in Japanese published unexamined patent application No. 28119/84 is known as a vari-focal lens system arranged to move only two lens groups. However, said known vari-focal lens system has a disadvantage that the variation of aberrations, especially the variation of astigmatism, is large when focused on an object at a short distance.
As one of problems related to optical systems to be used with an electronic image pick-up device, there is a problem of the back focal length. That is, the size of the image surface is small, i.e., about 1/4 to 1/5 of 35 mm format and, moreover, the electronic image pick-up device is being smaller and smaller year after year. Therefore, the focal length of the photographing lens system should be made short. As a result, the back focal length becomes necessarily short. Especially, between the rearmost surface (surface closest to the image surface) of the lens system and image surface, it is indispensable to reserve a space for inserting a thick quartz crystal filter or infrared ray eliminating filter as a low-pass filter. Moreover, when the single-lens reflex type is adopted, a space for inserting a quick return mirror or half mirror should be reserved in order to provide a finder optical system. In addition to the above, it may be necessary to insert a light measuring prism etc. Therefore, it is inconvenient when the back focal length is short. This means that it is especially required to provide a lens system whose back focal length is long compared with the focal length.